The present invention relates to a linear prediction speech coding method, in which a speech signal digitized as successive frames is subjected to analysis-by-synthesis in order to obtain, for each frame, quantization values of synthesis parameters allowing reconstruction of an estimate of the speech signal, the analysis-by-synthesis comprising short-term linear prediction of the speech signal in order to determine the coefficients of a short-term synthesis filter.
The present-day speech coders with low bit rate (typically 5 kbit/s for a sampling frequency of 8 kHz) yield their best performance on signals exhibiting a "telephone" spectrum, that is to say one in the 300-3400 Hz band and with pre-emphasis in the high frequencies. These spectral characteristics correspond to the IRS (Intermediate Reference System) template defined by the CCITT in Recommendation P48. This template has been defined for telephone handsets, both for input (microphone) and output (ear pieces).
However, it happens more and more frequently that the input signal of a speech coder exhibits a "flatter" spectrum, for example when a hands-free installation is used, employing a microphone with linear frequency response. Conventional vocoders are designed to be independent of the input with which they operate, and, besides, they are not informed of the characteristics of this input. If microphones with different characteristics are likely to be connected up to the vocoder, or more generally if the vocoder is likely to receive acoustic signals exhibiting different spectral characteristics, there are cases in which the vocoder is used in a sub-optimal manner.
In this context, a main purpose of the present invention is to improve a vocoder's performance, by rendering it less dependent on the spectral characteristics of the input signal.